pgsql: Refactor more the regression tests of pg_stat_statements

Refactor more the regression tests of pg_stat_statements

This commit expands more the refactoring of the regression tests of
pg_stat_statements, with tests moved out of pg_stat_statements.sql into
separate files. The following file structure is now used:
- select is mostly the former pg_stat_statements.sql, renamed.
- dml for INSERT/UPDATE/DELETE and MERGE
- user_activity, to test role-level checks and stat resets.
- wal, to check the WAL generation after some queries.

Like e8dbdb1, there is no change in terms of code coverage or results,
and this finishes the split I was aiming for in these tests. Most of
the tests used "test" of "pgss_test" as names for the tables used, these
are renamed to less generic names.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Y/7Y9U/y/keAW3qH(at)paquier(dot)xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0028e35a05ac71aebf28325c8ddb59a27cb63c0

Modified Files
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contrib/pg_stat_statements/Makefile | 4 +-
contrib/pg_stat_statements/expected/dml.out | 147 ++++
.../expected/pg_stat_statements.out | 768 ---------------------
contrib/pg_stat_statements/expected/select.out | 414 +++++++++++
.../pg_stat_statements/expected/user_activity.out | 199 ++++++
contrib/pg_stat_statements/expected/wal.out | 33 +
contrib/pg_stat_statements/meson.build | 5 +-
contrib/pg_stat_statements/sql/dml.sql | 76 ++
.../pg_stat_statements/sql/pg_stat_statements.sql | 300 --------
contrib/pg_stat_statements/sql/select.sql | 149 ++++
contrib/pg_stat_statements/sql/user_activity.sql | 65 ++
contrib/pg_stat_statements/sql/wal.sql | 20 +
12 files changed, 1109 insertions(+), 1071 deletions(-)